1. Field of the Invention
The present invention generally relates to a so-called "tuner" or a front end of a radio frequency (RF) receiver section integrated into a television transmitter/receiver or the like, and more particularly relates to a tuner structure with a reduced thickness and a cable modem tuner using the same.
2. Description of the Related Art
An exemplary conventional tuner structure is shown in FIG. 18, from which a shield cover has been omitted. As shown in FIG. 18, a circuit board 50 formed by mounting various circuit components (the respective circuit components are not shown) including resistors, capacitors, coils and transistors on a printed circuit board is incorporated into a chassis angle 51 (or the sides of a chassis disposed vertically with respect to the circuit board 50) having a folded metal plate structure. In general, a shield plate for isolating the internal circuits is also formed as an integral part of the chassis angle 51. A signal received through an antenna or the like is input through a connector 52 mounted onto a side of the chassis angle 51. In addition, terminals are also provided for supplying power, connecting control signals and retrieving output signals for the internal circuits. As the terminals, capacitor integrated terminals of a special type called "feedthrough capacitors (or feedthrough terminals)" 53 are used.
A tuner generally has an oscillator circuit therein. As a result, radio waves leak out of the tuner through the power supply terminals thereof and the like, thereby causing unnecessary radiation. In addition, when a noise is received through the terminals of the tuner, the noise is adversely mixed into the output of the tuner so that the output signal thereof is deteriorated. The feedthrough terminals (or feedthrough capacitors) 53 are provided for dealing with these problems.
The chassis angle 51, together with the shield cover, covers the body of the tuner and provides a satisfactory ground potential having a low impedance in an RF region. The feedthrough capacitor 53 has such a cross-sectional structure as that shown in FIG. 19. In the feedthrough capacitor 53, a terminal bar 53A extends through a dielectric 54 and an inner electrode 55 inside the dielectric 54 is satisfactorily electrically conductive with the terminal bar 53A via a solder layer 56 formed around the terminal bar 53A. On the other hand, an outer electrode 57 of the dielectric 54 is satisfactorily electrically conductive with the chassis angle 51 via a solder connection 58.
FIGS. 20A and 20B respectively show a method for evaluating the grounding effects for a case of using a feedthrough capacitor 53 and a case of using a chip capacitor 49 formed as a separate component. As shown in FIGS. 20A and 20B, the measurement is performed by connecting an RF signal source 202 (having a signal source impedance of 50.OMEGA.) to a level meter 204 (having a load impedance of 50.OMEGA.), inserting a capacitor to be tested between a signal line and a ground therebetween, and sweeping a frequency from 0 GHz to 3 GHz. The capacitor is connected to a pair of coaxial cables 48 having an impedance of 50.OMEGA. connected to the RF signal source 202 and the level meter 204, respectively.
FIGS. 21A and 21B respectively show the measurement results for the case of using the feedthrough capacitor 53 and the case of using the chip capacitor 49. As seen from these figures, in the case of using the feedthrough capacitor 53, more satisfactory attenuation characteristics are realized in the frequency region of about 0.5 GHz or higher. The reason is presumably as follows. Since the outer electrode 57 of the feedthrough capacitor 53 is directly connected to the chassis angle 51 of the tuner, no parasitic inductance is generated therebetween so that satisfactory grounding effects are realized.
On the other hand, in the case where a capacitor formed as a separate component (e.g., the chip capacitor 49) is provided between a terminal and a ground instead of using the feedthrough capacitor 53, the same effects as those of the feedthrough capacitor 53 can be surely attained in a low frequency region. However, since a copper case pattern inevitably exits when the electrode of the chip capacitor 49 is connected to a terminal or a ground and the capacitor 49 itself has a metal electrode pattern therein, these patterns function as parasitic inductances. As a result, in a high frequency region (e.g., 0.5 GHz or higher) where the influence of these parasitic inductances is not negligible, expected attenuation characteristics cannot be attained.
The tuner may be mounted onto a main substrate 64 either in a vertical mount fashion shown in FIGS. 22A and 22B or in a horizontal mount fashion shown in FIGS. 23A and 23B. FIG. 22A is a plan view showing a side on which an input terminal or the connector 52 has been mounted, while FIG. 22B is a plan view showing the side orthogonal to the side shown in FIG. 22A. On the other hand, FIG. 23A is a plan view showing the side on which an input terminal or the connector 52 has been mounted, while FIG. 23B is a plan view showing the side orthogonal to the side shown in FIG. 23A. The horizontal mount fashion shown in FIGS. 23A and 23B is used for a case where the space in the direction vertical to the substrate 64 on which the tuner is mounted is limited. For example, this type of mount is used for mounting a tuner onto an extended board of a personal computer.
FIG. 26 shows an exemplary internal structure of a conventional tuner structure to be mounted onto a substrate 64 in the horizontal mount fashion shown in FIGS. 23A and 23B. Chip components 60 and insert components 61 such as coils have been mounted onto a circuit board 50. A feedthrough capacitor 53 has been attached to a chassis angle 51 disposed vertically to the circuit board 50. The leg 53a of the feedthrough capacitor 53 is folded at a right angle, thereby electrically and mechanically connecting the feedthrough capacitor 53 to the main substrate 64. The outer sides of the tuner are covered with shield covers 65 and the tuner is electrically and mechanically connected to the main substrate 64 via the leg 51a of the chassis angle 51.
Next, a representative wiring process for assembling a tuner will be briefly described.
1) First, the chip components 60 and the insert components 61, such as coils which have been provisionally adhered to the circuit board 50 inside the tuner, are connected by a flow soldering method in which copper foil pattern surface of the circuit board 50 is immersed in a solder tank filled with molten solder. PA1 2) Second, the extra line portion of a reed line of each of the insert components 61 such as coils is cut off. PA1 1) Since the terminals 53a protrude from the chassis angle 51, the area of the main substrate 64 required for mounting the tuner thereon is increased. PA1 2) The terminals 53a protruding from the chassis angle 51 are so long that the positions of the terminals 53a possibly deviate because of the contact of the terminals 53a with something during the fabrication process of the tuner, during the transportation of the tuner or during the process of mounting the tuner onto the main substrate 64. Once the positions of the terminals 53a have deviated, it becomes difficult to mount the tuner onto the main substrate 64. In order to prevent such a deviation, it is necessary to provide a member for retaining the terminals 53a, thereby partially increasing the costs. PA1 3) When the feedthrough capacitors 53 are provided so as to protrude from a si,e of the chassis angle 51, an additional horizontal space should be reserved therefor. Since the space is added to the space reserved for a pawl for engaging the shield cover 65 with the chassis angle 51, the thickness of the tuner cannot be designed to be thin.
3) Finally, the circuit board 50 is inserted into the chassis angle 51 to which the feedthrough capacitors 53 and the input connector 52 have been attached. Then, the circuit board 50 is connected to chassis angle 51, the terminals of the feedthrough capacitors 53 are connected to the terminal of the input connector 52 by a similar flow soldering method to that described in 1).
However, a conventional tuner structure has the following problems.
For example, in the tuner structure of the horizontal mount type shown in FIGS. 23A and 23B, since the feedthrough capacitors 53 extend from a side of the chassis angle 51, the legs 53a thereof to be used as terminals are required to be folded. In the case of employing such a tuner structure, the following disadvantages cannot be prevented.
Furthermore, even if the problems about the terminals can be solved by any means, when a tuner uses a coaxial connector for connecting an input signal to the tuner, the size itself of the coaxial connector prevents the thickness of the tuner from being reduced. The size of a connector is standardized for convenience of general users. Thus, a tuner with a reduced thickness cannot be realized because of the limitation on the size of the connector. In other words, a connector of a special type having a reduced size cannot be used for reducing the thickness of a tuner.
In Japan, the United States and other countries, an input connector called an "F-type connector (or F contact tap)" is used as shown in FIG. 24. In FIG. 24, the size of the F-type connector 240 is defined as follows: the outer diameter of the threaded portion 66 is defined to be 9.4 mm.phi. and the outer diameter of the surface 68 to be contacted with the side of the chassis angle 51 11.0 mm.phi.. Thus, it is difficult to obtain a tuner structure having a thickness smaller than the outer diameter of 11.0 mm.phi. of the surface 68 to be contacted with the side of the chassis angle 51.
FIG. 25 is a plan view of a tuner structure to which a conventional F-type connector 240 has been mounted. In FIG. 25, the reference numeral 65 denotes an upper and a lower shield cover; 53 denotes a feedthrough capacitor; and 51 denotes a chassis angle. When the thicknesses of the chassis angle 51 and the tuner are denoted by C and D, respectively, the following relationship is satisfied as shown in FIG. 25:
(Outer diameter of the surface 68 of the F-type connector 240 to be contacted with the side of chassis angle 51: 11.0 mm.phi.)&lt;(Thickness C of the chassis angle 51)&lt;(Thickness D of the tuner).
For example, the thickness C and D are set at the following specific values: EQU C=about 12.3 mm to about 12.9 mm EQU D=about 14.0 mm to about 17.0 mm
The thickness C of the chassis angle 51 is designed to be larger than the outer diameter of the input connector 240 so as to secure the mounting strength of the input connector 240. Since a relatively thick coaxial cable is generally connected to the input connector 240, an input connector 240 having a weak mechanical strength is likely to deform the chassis angle 51. Furthermore, a drawing (for inflating the shield covers 65 and the like) is required to be performed so as to secure a sufficient strength for the shield covers 65 themselves and to surely engage inner shield plates (partition plates) of the chassis angle 51 with the shield covers 65 via pawls. Thus, the thickness of the tuner structure should also be increased because of the drawing and becomes rather larger than that of the input connector 240.
A conventional tuner structure and a cable modem tuner using the same have the following problems.
1) Downsizing a tuner structure
Since a cable modem is used as a peripheral component of a personal computer, such a modem is required to be downsized. However, since it is difficult to reduce the thickness of a tuner, the shape of a tuner should be modified.
2) Improving the strength of a chassis of a tuner structure
An F-type contact tap is used as an input terminal of a cable modem. When a coaxial cable is connected to the F-type contact tap, a load of 20 kg or more is applied to the F-type contact tap so that the chassis, to which the tuner input terminal is secured, is required to have a strength sufficiently strong to bear such a heavy load.
3) Improving the shielding effects of a cable modem tuner
A cable modem tuner should be connected to a cable line as a CATV appliance such that other equipment (such as a set top converter) is not affected by a spurious disturbance. Thus, as compared with a conventional TV tuner, the spurious level at the input terminal thereof should be improved.
Moreover, though a cable modem tuner to which a duplexor circuit has been integrated can bidirectionally transmit and receive data, a conventional TV tuner has no function of mixing an upstream signal and thus a duplexor circuit must be additionally provided therefor. When such a circuit is added, the filter thereof must be shielded.
Furthermore, since a cable modem tuner is a peripheral component of a personal computer, the tuner is disposed in the vicinity of the personal computer in most cases. As the cable modem is frequently exposed to digital noises generated from the personal computer, it is necessary to take some measures against the incoming disturbance signals.